1. Field of Invention
This invention relates to the field of surface finishing by lapping and more particularly to the field of an automatic ball to seat lapping method and apparatus.
2. Description of the Related Art
A production surface finishing process, commonly known to those skilled in the art as lapping, is used to provide a very smooth finish on various desired surfaces. Normally, lapping is associated with metal working or surface finishing of parts made of metals, but is not necessarily so limited. The smoothness of a surface finish is usually expressed in terms of surface roughness although the terms are often considered to be interchangeable. With known production lapping processes a surface roughness or finish in the range of 2 to 8 microinches, root mean square (rms) may be reliably achieved. As a relative guide to this surface finish measurement scale, a mirror normally has a surface roughness of 1 rms while the finest machined metal surfaces are finished in the range of 32 to 63 rms. As a surface roughness of 32 rms is more than adequate for most parts, surface treatment by lapping is normally utilized only on critical surfaces.
There are many devices and methods of measuring surface roughness known to those skilled in the art. Roughness measurements are usually expressed in microinches, either the mean arithmetic deviation from the average surface or the root mean square (rms), where a microinch is one millionth of an inch (0.000 001 or 10.sup.-6 in.).
As would be expected, the smoother the desired metal surface finish, the more expensive it is to obtain. And under the economic rule of the cost of diminishing returns, the last increment of surface roughness removed or smooth surface finish obtained greatly escalates the cost. The lapping process does not remove much metal or greatly change the dimensions of a part. Accordingly, the surfaces of parts to be finished by lapping are normally shaped very close to final tolerances before lapping begins to minimize the costs of lapping. Nevertheless, parts required to be finished by lapping are relatively expensive to manufacture.
While the lapping process may take a number of forms depending on the surface geometry of the parts involved, it is generally understood to require physical contact with relative motion, usually rotational, between the work piece or part being finished and a mating member, usually identified or called either a lap member or simply a lap, in the presence of a suitable lapping abrasive. The lap member may be either a special tool or the mating part itself. If the mating part is used as the lap member the two parts are considered to be dedicated or mate lapped together for final assembly purposes and are not thereafter considered interchangeable for assembly with others of the same type or part. Frequently, the special tooling lap member will be used initially for performing the course or rough lapping with the mating part being substituted as the lap member during the final precision stage of lapping. As used herein, lap or lap member will include either the mating part or special tooling lap.
A lapping fluid, usually liquid, having fine abrasive lapping particles suspended therein is continuously circulated between the lap and surface of the part being finished for lubrication and cooling. A predetermined lapping contact pressure is then applied to effect removal of the high spots of the part or parts having the surface being lapped. Due to the nature of the lapping process, this pressure is usually restricted to a low value to provide the desirable smooth finish. Because lapping time is related to the size of the abrasive particles and the contact pressure exerted, the metal removal process is slowly accomplished that further adds to the cost involved.
Production machines for lapping flat surfaces are well known and can achieve the normal highly polished, lapped range of surface finish with a flatness normally measured in helium light bands. Flat lapping machines are highly useful in forming flat fluid sealing metal surfaces for mechanical seals or flat metal-to-metal sealing surfaces or seats for certain types of gate valves. Their usefulness in forming fluid sealing seats in other types of valves has previously been limited due to the non-flat geometry of the sealing faces involved. Non-flat surface lapping has been in large part a manual operation.
Rotatable ball valves, as spherical plug valves are commonly known, are highly desirable for a number of reasons. They are quickly actuated (by a quarter turn of the metal ball flow closure element) between the open and closed position and provide a large, unobstructed central fluid flow passage through the valve and ball opening in the open condition or position. An additional advantage is the annular sealing area or arcuate face of the metal valve seat ring surrounding the central flow passage is protected from flow caused damage by continuous contact with the generally spherical outer surface of the ball in both the valve open and closed positions. This latter advantage is highly desirable in valve service where the fluid being controlled contains solid particles that abraded the seal surfaces.
Because of the cost and manufacturing difficulty in effecting a reliable metal-to-metal spherical contact annular fluid seal, such valve seat rings have usually been provided with a much more tolerant annular seal provided by a protruding soft resilient annular seal ring concentrically carried by the metal seat ring for contacting the ball. See for example, Tomlin U.S. Pat. No. 3,784,155. Such primary resilient soft seats are normally made of teflon, nylon or other polymeric material which limits the upper temperature range at which the rotatable ball valve will reliably seal. To assure fluid sealing operation in the event of a fire that will destroy the primary polymer soft seat, a normally inactive back up or secondary metal-to-metal, annular seal face may be provided on the seat ring for such emergency. Various published codes and standards also require such secondary metal-to-metal seats in rotatable ball valves.
The primary (or secondary) face seal of any rotatable ball valve is achieved by metal-to-metal contact between the arcuate or spherical shaped annular face of the valve seat and the outer spherical surface of the ball. This seal must be formed throughout the annular contact area as leakage will occur at any point not properly in sealing contact. To achieve this uniform metal contact for sealing, both the seat and ball surface must be finished to a high degree of smoothness and accuracy.
To achieve a reliable primary or secondary metal-to-metal seal between the spherical outer surface of the ball and the annular arcuate seat face, a very expensive manual or hand manipulated lapping process has commonly been employed. Such manual lapping operations produces erratic results as to the quality, uniformity and reliability of the fluid seal obtained and as to the time in which it is achieved; both are dependent on and functions of the skill of the person doing the manual lapping. For example, as the most highly skilled operator tires the lapping force or contact pressure is normally reduced or diminished and thereby increasing the lapping time.
To avoid the cost or expense of lapping rotatable ball valves having a primary or secondary metal-to-metal seal, various seat ring design approaches have been taken. For example, in Pennington U.S. Pat. No. 3,392,743, flexible annular metal sealing lips are provided on the seat rings to effect the annular primary metal-to-metal sealing engagement with the rotatable ball member. See also U.S. Pat. No. 4,126,295, to Natalizia for another variation of a rotatable ball valve where the entire metal seat ring is deformable.
Numerous automated approaches have been undertaken for enhancing spherical surface finish of a ball or spherical member. U.S. Pat. No. 537,071 to Fuller discloses an apparatus for polishing spherical balls, but not to achieve a metal-to-metal fluid seal with a mating seal of a valve.
Newman et al U.S. Pat. (No. 1,131,611) discloses a ball-to-seat lapping or grinding machine for single direction or ball check valve used in certain types of pumps. Unlike rotatable ball valves, the fluid sealing contact of the solid ball of a ball check valve is randomly positioned on the annular seat ring for sealing. This requires the entire spherical surface of the ball to be finished by mate lapping for effecting a desired fluid seal. The annular seal face of the seat ring is spring biased against the freely rotating solid ball during the mate lapping operation to obtain an accurate mate fit for fluid sealing on the spherical surfaces. No angular or other adjustment in position of either the seat or ball is provided during mate lapping.
U.S. Pat. No. 1,806,918 to Riggs is entitled "VALVE SEAT LAPPING MACHINE" and discloses an apparatus for simultaneously lapping solid balls and seats for ball valves. Two pairs of seats or collars are simultaneously lapped to a single solid ball employing a spring biasing on one seat to restrain the ball between the two seats. The seats are rotated on the relative angled axes provided by the shafts carrying the seats against the unrestrained or freely floating solid ball which is located between the rotating seats. The resulting outer surface motion of the ball is uncontrolled requiring a reversal or switch in position of the seat rings that doubles the lapping time required.
The patent to Capps U.S. Pat. (No. 1,950,785) also discloses a valve ball-to-seat lapping apparatus having an angular adjustment between the ball and the two rotating seats. The apparatus includes two opposed headstocks, each having a rotor upon which an inclined rotatable shaft is mounted. A seat is mounted on each shaft in a facing relationship for supporting the solid ball therebetween. The angular position or relationship of the two rotating seats is continuously changed to move the seats over all surface portions of the solid ball by a wobble motion. While the center of the ball is continuously fixed at the vertex of the changing seat angles and thus remains stationary the diameter ball spherical surface is free of any movement restraint. Motion of the two inclined shafts describe cones having vertexes aligned with shafts 8. As also disclosed in Capps the lapping operation for a ball and mating seat is commonly a manual activity with the ball pressed against the seat by the fingers of the workman.
In U.S. Pat. No. 2,075,216 to Mancuso multiple solid balls and seat rings are simultaneously lapped into a fluid-tight sealing relationship. This is achieved by an apparatus mounting a seat rings or seats in one of the plurality of chucks carried by a corresponding plurality of spindles. The solid ball is then positioned between the seat and the resilient tractive surface of the turntable and the spindle springs actuated. The turntable and planetary head are then selectively rotated to preferably mate lap the ball and seats. A planetary motion is given to the valve seat carried by the rotating spindle while insuring rotation of the ball. A yieldingly urging means or spring on the spindle urges the valve seats into lapping relation with the mated ball to obtain an even and accurate lapping. The ball is held in engagement with the seat by a flat tractive surface material carried on the rotating turntable which also imparts free lapping rotation to the ball.
Lichtenfeld U.S. Pat. No. 3,050,910 discloses a conventional rotating table flat lapping machine having a rotating flat lapping turn surface or plate 10. The parts having the flat surface to be lapped ar held against rotational movement with the table and are provided with a rotating or oscillatory movement relative to the table by rotating rings or lapping pots to obtain the desired flat lap surface.
The automatic flat lapping machine disclosed in Harris et al U.S. Pat. No. 3,111,791 is similar to and related to that of the Lichtenfeld patent discussed above. A counter rotation motion is induced in the work piece held in the lapping pots.
U.S. Pat. No. 3,111,789 to Harmon is entitled "SPHERE LAPPING MACHINE" and discloses apparatus for forming a precision gyroscope ball used in inertial navigational systems. The resulting ball is periodically tested for static balance, sphericity and diameter tolerances, but not for sealing or mate lapping with a valve seat. The solid sphere or ball is angularly supported by a pair of spring biased rotating work holders arranged on an intersecting axis that intersect at the center of the ball. To insure randomized lapping of the entire ball the vertical head shaft 13 angularly mounts the second shaft 22 carrying axially adjustable cast iron concave machine tool lap member for shaping the sphere. The lap 27 is continually driven in complex rotation about axis of shafts 13 and 22. Rotation of shaft 13 also rotates the angled shaft 22 about the ball while rotation of the shaft 22 simultaneously imparts a rotation to the lap.
The patent to Okano et al U.S. Pat. (No. 4,114,323) discloses a "DEVICE FOR AUTOMATICALLY LAPPING WEDGE-GATE VALVE SEATS". The resulting lap surfaces effecting the fluid seal are flat and not spherical.
U.S. Pat. No. 4,468,158 to Pearce et al discloses an apparatus for in place refinishing of the sealing surfaces of a top entry or loaded ball valve. During operation separate sealing rings are positioned in contact with the ball. The disclosed apparatus does not mate lap the ball to the refinished seat or shim for the seat rings.